Since photographic light-sensitive materials are generally composed of a base having an electrically insulating property and photographic layers, static charges are often accumulated when producing the photographic light-sensitive materials or using them by subjecting to contact friction between surfaces of the same or different kinds of material or separation thereof. The accumulated static charges cause various troubles. The most serious trouble is that the light-sensitive emulsion layer is exposed to light by discharge of accumulated static charges prior to development. This causes dot spots or resinous or feathery linear spots upon development of the photographic film. This phenomenon forms the so-called static mark, by which the commercial value of the photographic films is remarkably damaged or, sometimes, completely lost. For example, it is easily understood that static marks result in a dangerous judgment when they appear on medical or industrial X-ray films. Since this phenomenon becomes evident for the first time by carrying out development, it is a very troublesome problem. Further, the accumulated static changes causes secondary troubles, for example, dust may adhere to the surface of the films or uniform application of photographic layers to the films cannot be carried out.
Such static charges are often accumulated when producing photographic light-sensitive materials or using them, as described above. For example, during production, they are generated by contact friction between the photographic film and a roll or by separation of the base face and the emulsion face when winding or rewinding the photographic film. Further, they are generated in an automatic photographing apparatus by contact of the X-ray film with machine parts or with fluorescent sensitizing paper or separation therefrom. In addition, they are generated in contact with packing materials, etc. Generation of the static marks induced by accumulation of such static charges becomes rather substantial with increases in the sensitivity of photographic light-sensitive materials and increases in the processing rate. Particularly, in recent years, static marks are more easily generated, because the photographic light-sensitive materials have come to have high sensitivity and there are many opportunities for subjecting the materials to severe handling such as high speed application, high speed photographing or high speed automatic processing, etc.
In order to aid in eliminating the problems created by static electricity, antistatic agents are preferably added to the photographic light-sensitive materials. Antistatic agents utilized in the photographic light-sensitive materials must have different characteristics than antistatic agents conventionally used in other fields because there are various restrictions which are characteristic to photographic light-sensitive materials. Antistatic agents which can be utilized in photographic light-sensitive materials must not only have excellent antistatic properties but they must not have bad influences upon photographic properties such as sensitivity, fog, granularity or sharpness. Further, they must not have bad influences on the film strength of the photographic light-sensitive materials (namely, scratches are not easily formed by friction or scratching), they must not have bad influences on anti-adhesive properties (namely, the surface of the photographic light-sensitive material does not easily adhere to the surface of the photographic light-sensitive material or other materials), they must not promote fatigue of processing solutions for the photographic light-sensitive materials, or they must not reduce the adhesive strength between layers of the photographic light-sensitive materials. Accordingly, the application of antistatic agents to photographic light-sensitive materials is subjected to a number of restrictions.
One method of removing troubles due to static electricity comprises increasing the electrical conductivity of the surface of the photographic light-sensitive materials in order to disperse static charges in a short time prior and thus discharge accumulated charges.
Thus, methods of increasing electrical conductivity of the base in the photographic light-sensitive materials or various kinds of surface coating layer thereof have been proposed. Attempts have been made at utilizing various hygroscopic substances and water-soluble inorganic salts, certain kinds of surface active agents and polymers. For example, the use of polymers described in U.S. Pat. Nos. 2,882,157, 2,972,535, 3,062,785, 3,262,807, 3,514,291, 3,615,531, 3,753,716 and 3,938,999, etc., surface active agents described in U.S. Pat. Nos. 2,982,651, 3,428,456, 3,457,076, 3,454,625, 3,552,972 and 3,655,387, etc., and metal oxides and colloidal silica described in U.S. Pat. Nos. 3,062,700, 3,245,833 and 3,525,621, etc., are known.
However, it is very difficult to apply these substances to photographic light-sensitive materials, because they are particularly suited for one kind of film base or photographic composition. Accordingly, they produce good results when used with a specified film base or photographic emulsion or other photographic elements. However, they are useless for preventing static charges when used with different film bases and photographic elements, or they have an excellent antistatic property but have a bad influence upon photographic properties such as sensitivity of photographic emulsions, fog, granularity or sharpness, etc., or they have an excellent antistatic property just after production but the antistatic property deteriorates with the passage of time.
Nonionic surface active agents having one polyoxyethylene chain in a molecule are described in British Pat. No. 861,134 and German Pat. No. 1,422,309. These agents have excellent antistatic properties.
However, when they are applied to photographic light-sensitive materials the following problems occur: (1) they remarkably deteriorate sensitivity, (2) since their antistatic properties deteriorate with the passage of time, though they have good antistatic properties just after production, the antistatic properties of products become inferior when the products are used, and (3) when applied to X-ray sensitive materials, dotted or mesh-like uneven density (which is called "screen contamination") is formed on the sensitive materials after development, because the sensitive materials contact with sensitizing paper (fluorescent screen) when taking photographs. Accordingly, the value of the products is remarkably reduced and, sometimes, completely lost.
On the other hand, U.S. Pat. No. 3,850,641 has disclosed a method in which an ethylene oxide addition polymer of phenol-formaldehyde resin is applied as an antistatic agent for photographic light-sensitive materials. This polymer is synthesized by carrying out a polycondensation reaction of phenol derivatives and formaldehyde to form the so-called phenol-formaldehyde resin, and thereafter carrying out addition polymerization of ethylene oxide.
It is inevitable that the phenol-formaldehyde resin synthesized as described above is contaminated by unreacted phenol derivatives. Contamination of the unreacted phenol derivatives increases when synthesizing a resin having a lower degree of polymerization. Further, the process for removing unreacted phenol derivatives in the resin is very inconvenient. Even if the removal operation is repeated, it is very difficult to completely remove unreacted phenol derivatives. Accordingly, for practical purposes it is impossible to industrially produce phenol-formaldehyde resin which does not contain any unreacted phenol derivatives. In an ethylene oxide addition polymer of the phenol-formaldehyde resin contaminated with unreacted phenol derivatives, it is impossible to avoid various problems similar to those which occur with nonionic surface active agents having one polyoxyethylene chain in the molecule as described in British Pat. No. 861,134 and German Pat. No. 1,422,803. The problem occurs because these agents contain molecules having one polyoxyethylene chain in the molecule originated in the unreacted phenol derivatives in addition to molecules having many polyoxyethylene chains in the molecule.
Further, it is very difficult to obtain phenol-formaldehyde resin having a definite composition, because not only the content of unreacted phenol derivatives but also the average degree of polymerization or the distribution of degree of polymerization varies due to slight variations in conditions for synthesizing the resin. In addition, it is easily understood that, when the ethylene oxide addition polymer of phenol-formaldehyde resin is produced by addition polymerization of ethylene oxide, it is very difficult to control the polymer so as to have a definite composition to form an antistatic layer having a definite quality.
Moreover, other phenol resins such as phenol-acetaldehyde resin or phenol-furfural resin, etc., have similar problems.